Laws of chemical combination

Correct answer: B. Law of multiple proportions

CO and CO₂ contain the same two elements in different proportions. For a fixed 12 g of carbon, they contain 16 g and 32 g of oxygen, respectively; 16:32 simplifies to 1:2, a simple whole-number ratio. This illustrates the law of multiple proportions.

Correct answer: A. The volumes of the reacting gases

Gay-Lussac's law of combining volumes states that, at the same temperature and pressure, reacting gases and gaseous products have volumes in simple whole-number ratios. Thus, product-gas volumes are compared with the volumes of the reacting gases.

Correct answer: A. Law of gaseous volumes

Gay-Lussac's law of combining volumes describes simple whole-number ratios between the volumes of reacting gases and gaseous products at the same temperature and pressure. The other listed laws concern mass relationships.

Correct answer: A. Law of definite proportions

The law of definite proportions states that a pure compound always contains its constituent elements in the same fixed ratio by mass, regardless of the sample's source or size.

Correct answer: B. Law of multiple proportions

The correct answer is B, the law of multiple proportions. When the same two elements form different compounds, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio.

Correct answer: A. Law of conservation of mass

The correct answer is A, the law of conservation of mass. In an ordinary chemical reaction, atoms are rearranged into new substances rather than created or destroyed, so the total mass remains unchanged in a closed system.

Correct answer: B. Law of definite proportions

The correct answer is B, the law of definite proportions. A pure compound always contains the same elements in a fixed mass ratio, regardless of the sample taken.

Correct answer: A. Law of definite proportions

The correct answer is A, the law of definite proportions. A mixture can have a variable composition, whereas this law describes the fixed elemental mass ratio in a pure compound.

Correct answer: A. Accurate experimental measurements

The correct answer is A, accurate experimental measurements. These laws describe quantitative relationships in chemical reactions, established by measuring quantities such as mass and, for gases, volume.

Correct answer: A. Substances that start the reaction

The direct answer is A: reactants are the substances that are present at the beginning and take part in a chemical reaction. In a chemical equation, reactants are written on the left side, and products are written on the right side. During the reaction, atoms are rearranged: the reactants change into new substances called products. The law of conservation of mass says that atoms are not created or destroyed in the reaction, so the total mass of reactants equals the total mass of products in a closed system. A is correct because it identifies the starting substances. B is wrong because substances formed after the reaction are products, not reactants. C is wrong because reactants may be solids, liquids, gases or solutions; they are not only gases. D is wrong because colour is a property or observation, not a substance. A useful example is hydrogen and oxygen reacting to form water: hydrogen and oxygen are reactants, while water is the product. Memory cue: reactants react first; products are produced afterward.

Correct answer: A. Substances formed in a reaction

The correct answer is A. Products are the substances formed in a chemical reaction; in a closed system, their total mass equals the total mass of the reactants.

Correct answer: B. 20 g

The correct answer is B, 20 g. By the law of conservation of mass, total mass is unchanged in a closed system: product mass = reactant mass = 20 g.

Correct answer: C. 12 grams

The direct answer is option C: 12 grams. In a closed system, matter is not lost or gained during a chemical reaction. The total mass of all reactants before the reaction equals the total mass of all products after it, provided the system is closed. Here the reactant masses are 5 grams and 7 grams. Step 1: write both masses. Step 2: add them: 5 + 7 = 12 grams. Step 3: therefore, the total product mass is 12 grams. Option A, 2 grams, is the difference between 7 and 5, not the total. Option B, 7 grams, ignores the 5-gram reactant. Option C, 12 grams, correctly applies conservation of mass. Option D, 35 grams, incorrectly multiplies 5 by 7; multiplication is not required. Remember: in a closed reaction, add all reactant masses to obtain total product mass.

Correct answer: A. Masses of elements

The direct answer is A: the masses of the elements. The law of definite proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of where the sample came from or how it was prepared. For example, pure water contains hydrogen and oxygen in a fixed mass ratio of 1:8. The ratio concerns the elements forming the compound, not their colours, smells or classroom labels. Option A is correct because it names the measured quantities in this law. Option B is wrong because container colour has no necessary chemical relationship with composition. Option C is wrong because odour is a sensory property, not the fixed quantitative relationship stated by the law. Option D is irrelevant because class numbers are administrative labels and have nothing to do with chemical composition. A useful distinction is that definite proportions concerns one pure compound and its fixed mass composition; it does not say that every mixture has a fixed composition. Memory cue: definite proportion = definite mass ratio of elements in a pure compound.

Correct answer: A. A simple whole-number ratio

The correct answer is A. For two elements forming more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio.

Correct answer: A. Gases

The correct answer is A, gases. Gay-Lussac's law of combining volumes relates the volumes of reacting gases and gaseous products, measured at the same temperature and pressure.

Correct answer: A. Law of gaseous volumes

The direct answer is option A: the law of gaseous volumes. This law, associated with Gay-Lussac, states that when gases react, their volumes can be in simple whole-number ratios when measured at the same temperature and pressure. The key word is volume, not mass. Option A is correct because it directly describes simple volume ratios of reacting gases. Option B, the law of definite proportions, says that a pure compound always contains the same elements in the same fixed mass ratio; it concerns composition by mass. Option C, the law of multiple proportions, compares the different masses of one element that combine with a fixed mass of another element in different compounds. Option D, conservation of mass, says total mass before and after a reaction remains equal. It does not specifically state gas-volume ratios. Thus, whenever the question highlights simple ratios of gaseous reactant and product volumes under the same conditions, choose the law of gaseous volumes. Exam cue: volume plus gases plus same temperature and pressure points to Gay-Lussac's law.

Correct answer: A. Law of constant composition

The law of definite proportions states that the constituent elements of a pure chemical compound are always present in a fixed ratio by mass. Hence, it is also called the law of constant composition. The law of multiple proportions applies when two elements form more than one compound. Exam tip: remember that “definite proportions” and “constant composition” are two names for the same law.

Correct answer: A. It remains the same

The composition of a pure compound is constant, regardless of its source. Its elements are present in a fixed proportion by mass, as stated by the law of definite proportions.

Correct answer: A. Atoms combine in simple whole-number ratios

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios. This supports the idea that atoms combine in whole-number ratios.

Correct answer: A. The mass relationship among three elements

This law compares the masses of two elements that combine separately with a fixed mass of a third element. It is therefore identified by a mass relationship involving three elements.

Correct answer: B. Total mass is destroyed in a reaction

The law of conservation of mass says that total mass remains constant during a chemical reaction in a closed system. Therefore, the claim that mass is destroyed contradicts the law.

Correct answer: D. The elements in a pure compound can be present in any ratio

A pure compound has a fixed composition by mass, so its elements cannot be present in an arbitrary ratio. Variable proportions are possible in mixtures, not in a pure compound.

Correct answer: A. Gas volumes are in simple ratio at same temperature and pressure

The direct answer is option A. The law of gaseous volumes says that reacting gas volumes and gaseous product volumes are related in simple whole-number ratios when all volumes are measured at the same temperature and pressure. The reasoning is: first identify that the statement is about gases and their volumes; next check the required conditions, namely equal temperature and pressure; then match it with Gay-Lussac's law. Option A includes both the correct quantity, gas volume, and the correct conditions, so it works. Option B says that lengths of solids are always equal. This is unrelated and is also not generally true. Option C refers to the taste of liquids, which has no connection with a chemical law of gas volumes. Option D says every mixture has a fixed colour; mixtures may have different colours and colour is not the measured quantity in this law. Therefore A is the only scientifically relevant statement. A useful exam reminder is that a volume-ratio law needs gases and specified common conditions; do not select an option merely because it mentions a physical property.

Correct answer: A. Whether the data concern mass ratios or volumes of gases

Different laws address different kinds of relationships. Identifying whether the given information concerns mass ratios or gas volumes helps you choose the relevant law.